1. Field of the Invention
The present invention relates to focus control technology.
2. Description of the Related Art
One of focus control techniques used with electronic cameras is the so-called contrast detection scheme, in which focus control is performed by using a focus lens position that maximizes the high-frequency components of a luminance signal of an object image that is formed on an image sensing sensor as an in-focus position at which a desired object image enters an in-focus state. An abundance of high-frequency components in the luminance signal of an object image indicates that the object image has a high contrast, and therefore, is in better focus compared to a case where the amount of high-frequency components is low. For instance, refer to NHK Technical Research Report (1965, Volume 17, Issue 1, Serial Number 86, pp. 21-37). With such a contrast detection scheme, the focus lens is moved to acquire high-frequency components of object images at a plurality of different focus lens positions (scanning operation), and is then moved to a position that maximizes the high-frequency components.
However, due to the limited amount of time for performing scanning operations in so-called sequential image sensing with short image sensing intervals, focusing is locked at the in-focus position at the start of sequential image sensing, or the moving range of the focus lens for scanning operations in the interval between one image sensing and the next image sensing in the sequential image sensing is limited.
In addition to the sequential image sensing, performing AF control every time image sensing is performed has a problem in that a release time lag occurs between the time when image sensing commencement request operations are performed by an operator, and the time when image sensing is actually performed. In consideration thereof, when it is desirable to perform image sensing while prioritizing release time speed, since the amount of time for performing scanning operations is limited, a method is proposed in which focusing is locked at the in-focus position of the last image sensing (For instance, refer to Japanese Patent Application Laid-Open No. 2003-018450). Additionally, a method is proposed for limiting the moving range of a focus lens during scanning operations. (For instance, refer to Japanese Patent Application Laid-Open No. 2001-208959).
Furthermore, a method is proposed for bringing a moving object into focus in which, when performing focus control for the next image sensing, the time required for focus control is reduced by setting the moving range of a focus lens to be equally distributed on both sides of the present in-focus position. For instance, refer to Japanese Patent Application Laid-Open No. 2004-102135.
Moreover, since image sensing is sequentially performed at short time intervals during sequential image sensing, in many cases, the object is moving in the same direction among consecutive image sensings. Therefore, it is possible to partially predict the in-focus position at the time of the next image sensing, based on the direction of movement of the object up to the time of the previous image sensing; or based on the driving direction, i.e., the direction of movement of the in-focus position, up to the time of the previous image sensing. A method is proposed which utilizes the characteristic to vary distribution of the driving range of a focus lens using the present in-focus position as a reference. For instance, refer to Japanese Patent Laid-Open No. 2002-122773.
The example shown in FIGS. 16A to 16C shows in-focus positions at three consecutive image sensings. In FIG. 16C, based on the directions of movement of the in-focus position at the time of the second last image sensing, in FIG. 16A, and the last image sensing, in FIG. 16B, and, using the in-focus position at the last image sensing, in FIG. 16B, as reference, the far-side and near-side distributions (SC1, SC2) of the scan range with respect to the in-focus position at the last image sensing are arranged so as to differ from each other. In other words, the far- or near-side distribution extending toward the same direction as the direction of movement up to the last image sensing is set wider than the other.
By thus varying distributions of the driving range of a focus lens based on the direction of movement of the in-focus position up to the last image sensing, it is now possible to drive the focus lens in an effective manner and speed up focusing operations.
However, as shown by the preceding conventional example, performing focus control while uniformly prioritizing the reduction of release time lag has a risk in that the background or other objects that is not the main object may be brought into focus, or that erroneous focusing will be performed. In particular, since the conventional example of sequential image sensing described above merely set one of the far- and near-side scan range distributions wider than the other without changing the reference position (i.e., the in-focus position at the last image sensing) thereof, the example is incapable of dealing with an object that gradually increases its image plane movement speed. More specifically, since the image plane movement speed increases at an accelerating pace for an ordinary object that approaches at constant speed, it may be difficult for the conventional example to bring an object moving in such an ordinary manner into focus.
Furthermore, with the conventional method, in cases where, for instance, an object with a significantly high contrast that is not a main object exists in the background or the like, and the object is within the focus detection area, detection results will reflect an in-focus position that relies on the high-frequency components of a luminance signal other than that of the main object. Moreover, the following problem exists when an object such as the background that is not the desired object is erroneously brought into focus. Specifically, there are cases where the in-focus position of the main object will not enter the driving range of the focus lens even when the far- or near-side of distribution extending toward the same direction as the direction of movement up to the last image sensing is set wider than the other, and an object such as the background will remain in focus.